1. Field
The present disclosure relates to a method and a system that is configured to minimize two-dimensional image quality non-uniformities on printed documents.
2. Description of Related Art
Two-dimensional image quality non-uniformities affect the performance of an image printing system. Two such two-dimensional image quality non-uniformities that are known to originate in the image printing system include noise mid-frequency (often referred to as “mottle”) and reload.
Many image printing devices use donor rolls to transfer toner to an image bearing surface for developing an image thereon. These donor rolls generally accumulate toner as they rotate. After transferring toner to an image or a portion of an image, the donor roll “reloads” with toner as it rotates. Depending on the previous image content or portion of an image being developed, the donor roll may not be able to accumulate a sufficient level of toner to properly develop the current image. This inability to fully reload the donor roll causes the later drawn image or portion of an image to have an area lighter than it should be.
This failure to complete reloading of the donor roll in one revolution results in an image quality non-uniformity called reload error. The reload error is referred to as a depletion of toner on the donor roll of a toner development system. The reload error can occur in any device using a donor roll.
For example, the reload defect occurs where the structure of an image from one revolution of the donor roll is visible in the image printed by the next successive donor roll revolution, a phenomenon known in the art as “ghosting”. At locations on the donor roll where previous images were located, the level of toner may be lower than desired. This causes an undesirable lightening of parts of an image, depending on what was imaged earlier. One area where reload error may have a significant effect is in color calibration systems.
Irregular two-dimensional variations caused by various sources of noise in the printing process can form graininess or mottle in the image. For example, in an electrophotographic system, graininess and mottle are usually found in and caused by the development subsystem, and mottle can be enhanced by the incomplete transfer of toner to substrate. The mottle is often characterized by the non-uniform printing or coloring of an image. Both of these are two-dimensional variations in gray level, which take the appearance of dots or small irregular shapes. Graininess is similar to mottle but the variations are smaller in size.
These two-dimensional image quality non-uniformities (e.g., mid-frequency image noise (mottle) and reload) are known to originate in the toner development system of the image printing system and, in the case of mottle, can be magnified by subsequent xerographic sub-systems. The degradation in mottle and reload performance often results in unsatisfied customers, and unscheduled service actions (i.e. replacement of the developer material) that are both costly and unproductive to the manufacturer/service provider and customers alike.
Other than the magnetic roll bias, all other toner development system parameters (i.e. donor to magnetic roll AC voltage, toner concentration, magnetic roll speed, etc.) generally have fixed values in the prior systems, which are determined through optimization testing across various noise inputs. Through this testing, performance is often traded off against sub-system latitude, shortchanging maximum achievable image quality performance.
U.S. Pat. No. 7,236,711, herein incorporated by reference, discloses a method for identifying specific transfer defects in a xerographic print engine using residual mass. These specific defects may include mottle, graininess, streaks, or point deletions. A full width array is used as a residual mass sensor. Upon identification, a closed-loop control of the transfer process is performed taking into account the identified defect types, as well as their magnitudes, to correct or compensate for the defects. This patent, however, senses the residual mass remaining on a photoreceptor, or other substrate, surface after transfer process (e.g., the transfer of toner to a media) in a Xerographic process. In contrast, the present disclosure scans the developed toner images on an image bearing surface to detect the two-dimensional image quality non-uniformities of the toner image on the image bearing surface.
Specifically, the present disclosure proposes a method and a system to sense and subsequently minimize toner development system related two-dimensional image quality non-uniformities on printed documents through closed loop control of the appropriate toner development system parameters. The closed loop control of the appropriate toner development system parameters will ensure maximum two-dimensional image quality performance (e.g., optimal mottle and reload performance) consistency.